MastoAPI transition: notifications

Should not rename everything nor bring tons of refactoring to avoid merge conflicts and ease the review and testing.

For every missing BE functionality there should be mention of it in pleroma#674, probably create a new task too.

Code from mastoapi-exploration branch could be backported/separated or used as a reference.

Right now MastoAPI is missing is_seen property present in QvitterAPI, it is mentioned in pleroma#674. This isn't the end of the world and could be easily worked around but it would bring regression of #380 (closed)
Another possible regression could be that notifications in MastoAPI do not have "x favorited status y" and probably repeats are handled differently too, so Pleorma-FE won't increment post counters from notifications anymore automatically, but this is a such minor case it could be worked around separately if it's true.